A paper production process is roughly divided into a pre-process (pulping process) in which pulp is produced from wood, waste paper, etc., and a post-process (papermaking process) in which sheet-like paper is obtained from pulp. In the pulping process, for example, wood chips are cooked under strong alkaline conditions, and then the obtained pulp is bleached with chlorine, hypochlorite, chlorine dioxide, oxygen, ozone, etc. to improve whiteness. Then, dirt containing degraded products generated during bleaching is washed away by alkali extraction to obtain bleached pulp. In the papermaking process, pulp is spread in a sheet form in a wire part, and is then dehydrated in a press part and dried in a dryer part.
Wood as a raw material of paper contains oils and fats, fatty acids, terpenes, steroids, natural resins, gum-like substances, etc., and these substances remain also in pulp obtained in the pulping process as micron-sized colloidal particles. These colloidal particles agglomerate to form larger particles called “pitch”.
Adhesion of pitch to flow pipes, instruments, fan pumps, chests, wires, felts, rolls, etc. in paper production equipment leads to contamination of the equipment. Further, re-adhesion of pitch that has adhered to the equipment to pulp or paper leads to a reduction in paper quality due to stains and defects or to breakage of paper. In order to remove adhered/deposited pitch, it is necessary to stop production lines, which leads to a reduction in productivity and workability. Various pitch control agents are used in the pulping process and the papermaking process in order to prevent such a reduction in productivity or paper quality caused by pitch trouble.
As a pitch control method, a method is known in which substances responsible for pitch (micro-pitch) are adsorbed to a porous inorganic material such as talc or clay, and the porous inorganic material is mixed into paper to remove micro-pitch out of a system. However, such a porous inorganic material is poor in the ability to adsorb micro-pitch and to fix to paper, which leads to pollution of white water and deposition of sludge. Further, the porous inorganic material may cause wear of or damage to papermaking wires.
A method in which a cationic polymer or an amphoteric polymer is added as a pitch control agent to pulp slurry has been proposed as a method for discharging micro-pitch out of a system by fixing micro-pitch to pulp or paper in a state where the micro-pitch is dispersed by preventing its agglomeration. For example, Patent Document 1 and Patent Document 2 disclose methods in which a specific amphoteric polymer is added as a pitch control agent to pulp slurry in the process of papermaking from waste paper pulp.